Modern imagesetters and platesetters utilize optical scanners to write or record images for subsequent reproduction or to read a prerecorded image at a predefined resolution rate. Such scanners may write or record images on or read prerecorded images from various media including photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials, aluminum or other metal base printing plate, or other type media. The medium is typically mounted on an imaging surface which may be planar or curved and then scanned with an optical beam.
The primary components of modern imagesetting and platesetting systems include an image processor to generate and/or edit a digital image, and a raster image processor (RIP) for converting data signals from the image processor into signals which can be understood by a controller which controls the scanning of the optical beam by the imagesetter or platesetter.
The imagesetter or platesetter itself typically includes a scan assembly which is often supported so as to be movable within a drum cylinder in which the recording or recorded medium is mounted. The controller, in accordance with the signals from the RIP and its own programmed instructions, generates signals to control the optical scanning so as to write images on or read images from the medium mounted within the drum cylinder by scanning one or more optical beams over the inside circumference of the drum cylinder while the cylinder itself remains fixed.
A typical scan assembly of a cylindrical drum type imaging system includes a radiation source such as a laser diode or other optical beam generator, one or more lenses to precisely focus the optical beam onto an image plane via a spin mirror or other optical device which scans the light beam over the inside circumference of the drum cylinder, as will be well understood by those skilled in the art.
High resolution imagesetters and platesetters require precise focusing to obtain output images free of undesirable artifacts generally known as banding and dot gain/loss. Banding and/or dot gain can result from small variations in beam spot size on the material or image receiving surface. Even though a slight deviation in the system focus may not have a significant effect on the beam spot size at the image receiving surface, variation of spot size caused by dynamic variations of focus produces visible artifacts on the output image. If the system focus error is eliminated, the affect of the dynamic variations of focus is minimized and a source of banding accordingly reduced.
Large format imagers have difficulty maintaining focus because of changes in the temperature in the support members to which optical system components and the media to be imaged are mounted. In conventional imaging systems, the focus is set at the factory or during initial installation, or during initiation of operations. The focus is preferably established based upon the normal operating temperature of the imaging system; however, the actual operating conditions may differ from what might generally be characterized as normal operating conditions due to the general environmental conditions in which the imaging system is operating. Further, the normal operating conditions will typically change as the imaging system ages. Further still, certain components of the scanning assembly may be replaced during the life of the imaging system and the new components may react differently at the normal operating conditions than did the replaced components. Additionally, the imaging system may be operated from time to time immediately after start up, and therefore imaging may be performed prior to the system actually reaching its normal operating condition. Any of these conditions can result in a system focus error which will cause a significant degradation in the quality of the imaging.
Recently, mechanisms have been developed which allow for dynamic adjustment of the system focus within imagesetting and platesetting systems. For example, U.S. application Ser. No. 08/373,712, which is commonly assigned to the assignee of the present application, discloses a focus mechanism which allows real time focus adjustment in optical imaging systems. As described therein, the adjustment mechanism allows precise adjustment of the focus for various thicknesses of imaging material. However, a need remains for an imaging system in which the optical beam focus is adjusted to correct for focus errors caused by variations in the system operating conditions.